Method of manufacturing indirectly heated cathodes



July 2, 1957 METHOD FURNACE H. BENDER 2,798,010

OF MANUFACTURING INDIRECTLY HEATED CATHODES Filed May 25, 1955 FURNACEINVENTOR. HARRY BENDER BY MQ%\ Unite States Patent Office 2,798,010Patented July 2, 1957 METHOD Q15 MANUFACTURING INDIRECTLY HEATEDCATHODES Harry Bender, Albertson, N. Y., assignor to Syivania ElectricProducts Inc., a corporation of Massachusetts ApplicationMay 23, 1955,fierialldo; 516,489

4 Claims. (Cl. 117-==-217)' My invention relates to improved electrontubes. and methods for making the same.

Many types of conventional electron tubes employ indirectly heatedcathode structures. Typically, such' a structure comprises a hollownickel cylinder or sleeve, the outside surface of which is coated withelectron emissive material, and a suitably insulated tungsten heaterelement is mounted within the sleeve. Heat radiated by the element whenit is electrically energized impinges on the sleeve; when the sleeveattains a suitable operating temperature (on the order of 800 C.),electrons are emitted. from the surface of the electron emissivecoating.

This type of structure, due to the number and complexity of thecomponents, is relatively expensive to produce in quantity. Moreover,the efliciency of heat transfer is low. As a result, a relativelylongwarmup or heating period is required. Asa further result, theoperating temperature of the heater is much higher than that of the,sleeve, since the useful life of the element decreases rapidly as itsoperating temperature increases, this temperature differential isobviously undesirable.

Accordingly, it is an object of the invention to simplify theconstruction and improve theheat transfer efficiency of indirectlyheated cathode structures.

Another object is to provide a new process for producing indirectlyheated cathode structures simply and inexpensively.

Still another object is to' provide a new and improved indirectly heatedcathode structure characterized by a short warm-up period and anextended useful life.

These and other objects of my invention will either be explained or"will become apparent hereinafter.

In my invention, a refractory metallic heater element, formed forexample, from tungsten, molybdenum or alloys thereof, is first coatedwith a layer of ceramic material such as aluminum oxide or other ceramiccharacterized by low electrical conductivity at elevated temperatures.This layer is sintered to bond the layer to itself and to the heater. Asecond layer of metallic material such as nickel is then applied overthe oxide layer. The composite structure is then heated to sinter andbond the metallic layer to itself and to the first layer. Finally athird layer of electron emissive material such as a mixture of thecarbonates of barium, calcium and strontium is then applied over thesecond layer and sintered to form the completed cathode.

The aluminum oxide coating of this structure is somewhat porous; ifsuitable precautions are not taken, the pores can be filled with metalsor other compounds; as a result, undesired electrical conducting pathscan be formed between the outer layer and the heater element. However, Ihave discovered that when the size of the metallic particles issufiiciently large, these particles will seal off (and not penetrate)the oxide layer, and leakage no longer presents a problem. I havefurther discovered that when the oxide layer is covered with a thinprotective coating, for example a lacquer coating, to seal the poresbefore the metallic layer is applied, the size of the metallie particlesis not critical and again the leakage problem, is solved. In either casean indirectly heated cathode characterized by low heater cathode leakageis produced.

These coatings can either be applied electrophoretically or by aconventional drag-cup process well known to the art. Either techniqueresults in thin coatings. Consequently, the emission surface issubstantially the same size as the heater surface and is in closeproximity thereto. Therefore, the warm-up period of the cathode issharply reduced. Further, the temperature differential between theheater element and the emissive surface is. also-reduced and the usefullife of thecathode is correspondingly increased.

An illustrative embodiment of my invention will now be described indetail with reference to the accompanying, drawings wherein:

Fig. 1 shows one form of an indirectly heated structure in accordancewith tlie'invention; and

Fig. 2 illustrates diagrammatically a heater element coating process forproducing the structure.

Referring now to the drawings, the indirectly heated cathode of Fig. 1comprises a heater element, for example a tungsten wire 1, clad with alayer 2 of ceramic material such as aluminum. A second layer 3 ofmetallic material such as nickel surrounds the first layer and a thirdlayer 4 of electron emissive material such. as a mixture of thecarbonates of barium, strontium and calcium covers the second layer.Each of these layers is securely bonded to each other, and the firstlayer is likewise securely bonded to the heater element; The ends ofwire 1 are exposed for suitable electrical connection. A nickel tab 5 iswelded or otherwise connected to the nickel layer and serves as acathode return connection.

Fig. 2 illustrates a process for producing the structure of Fig. l. Thetungsten wire is unreeled from a supply spool and is passed under pulley101 and over pulley 1&2 into pulley 103 of drag cup 104. This cupcontains a liquid dispersion'of aluminum oxide and suitable binderparticles. The wire leaving drag cup 104 is loosely coated with a layer2 of Alundum particles. The oxide coated wire is then passed through afurnace 105 wherein it is heated to a temperature on the order of 1800"C. in an oxygen free atmosphere containing argon or other inert gas or areducing agent such as moist hydrogen to sinter the oxide particles toeach other and to the wire. This coating is somewhat porous. In order toprevent substantial leakage, the Alundum coated wire is then passed overpulleys 106 and 167 onto pulley 108 of drag cup 1&9. This cup contains aconventional lacquer.

The lacquered wire is then passed under pulley 110 and over pulley 111onto pulley 112 of drag cup 113. This cup contains a liquid dispersionof nickel particles and a suitable binder. The wire leaving drag cup 13then is coated with a sintered layer 2, a thin layer of lacquer, and aloose layer of nickel particles. The wire is then fed through a secondfurnace 114 where it is heated in an oxygen free atmosphere to atemperature on the order of 1500 C. As a result, the lacquer coating isvaporized and a sintered nickel layer is formed and bonded to the oxidelayer. The nickel coating seals off but does not penetrate the pores ofthe oxide layer. I do not fully understand why this action occurs, but Ibelieve that the nickel particles will tend to sinter and bond to eachother to some extent before the lacquer is completely vaporized, andthis initial sintering prevents nickel particles from falling into thepores. Alternatively, when the nickel particles are sufficiently largeto seal off the oxide pores, the lacquer application step can beeliminated.

The wire leaving furnace 114 is then passed over pulley 115 and underpulley 116 onto pulley 117 of drag cup 118. This cup contains a slurryof alkaline earth carbonates. The wire leaving drag cup 118 then hasbeen coated with a sintered aluminum oxide layer 2, a sintered nickellayer 3, and a loose carbonate layer 4. The wire is then fed through athird furnace 119 wherein it is heated in an atmosphere of carbondioxide to sinter the particles of layer 4 to each other and to thenickel layer to form the completed structure. The coated wire leavingfurnace is then wound over take-up spool 120.

Two or more drag cups containing the same dispersion can be arranged inseries to build up the thickness of any particular layer to any valuedesired. Thus, for example, successive layers of aluminum oxide can beapplied as necessary. However, in order to insure that each Alundumlayer has sufiicient adherence to permit the addi tion of a subsequentAlundum layer, it is necessary to interpose heating stations betweenadjacent aluminum oxide containing cups to dry the binder and thusobtain the desired amount of adherence. After the overall thickness hasbeen increased to the desired value, the entire layer is sintered in themanner previously indicated.

I have found that in order to form an acceptable metallic layer using asingle cup, the ratio by weight of liquids to solids in the applicabledispersion must be on the order of 4/5. Either aqueous-base orlacquer-base dispersions can be used. A typical aqueous base dispersionhas the following composition:

A typical lacquer base dispersion has the following composition:

Lacquer 50 cc. Amyl acetate 100 mls. Nickel particles 100 gms.

When the preliminary lacquer coating technique is not used, the particlesize of the nickel is critical for leakage purposes as indicatedpreviously. I have found that nickel flake powder of the type identifiedas MD75O and manufactured commercially by Metals Disintegrating Co. hasthe proper particle classification for this purpose; i. e. these powderswill pass through a 325 mesh screen.

While I have shown and pointed out and described my invention in onepreferred embodiment, it will be apparent to those skilled in the artthat many modifications can be made within the scope and sphere of myinvention as defined in the claims which follow.

What is claimed is:

1. In a method of manufacturing an indirectly heated cathode, the stepsof depositing ceramic particles having low electrical conductivity outof liquid media onto the surface of a refractory metallic heaterelement; heating the resultant structure in an oxygen free atmosphereuntil a sintered somewhat porous ceramic layer well bonded to saidsurface is formed; coating said ceramic layer with lacquer to seal offthe pores thereof; depositing metallic particles out of liquid mediaonto the ceramic layer; and heating the resulting structure in an oxygenfree atmosphere until a sintered metallic sheath is formed, said sheathbeing well bonded to said ceramic layer and sealing off the poresthereof.

2. In the method as set forth in claim 1, the further steps ofdepositing particles of electron emissive material out of liquid mediaonto the metallic sheath; and heating the resultant structure in anoxygen free atmosphere until a sintered electron emissive layer wellbonded to' the metallic sheath is formed.

3. In a method for manufacturing an indirectly heated cathode, the stepsof depositing ceramic material of low electrical conductivity out ofliquid media onto the surface of a refractory metallic heater element;sintering said ceramic coated element to form a porous ceramic layerwell bonded to said surface; coating said ceramic layer with lacquer toseal olf said pores; depositing metallic particles out of liquidmedia-onto the lacquer coated element; and sintering the resultantstructure in an oxygen free atmosphere to form a metallic sheath wellbonded to said ceramic layer, the pores of said layer being free fromsaid metallic particles.

4. In a method for manufacturing an indirectly heated cathode from arefractory metallic heater element coated with a porous layer of ceramicmaterial characterized by low thermal conductivity at elevatedtemperatures, the steps of depositing metallic particles out of liquidmedia onto the surface of said ceramic layer, the size of said particlesbeing related to the size of the pores in said layer in such manner thatthe particles seal off and do not penetrate said pores; and sinteringthe resultant structure until a sintered metallic sheath well bonded tosaid porous layer is formed.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A METHOD OF MANUFACTURING AN INDIRECTLY HEATED CATHODE, THE STEPSOF DEPOSITING CERAMIC PARTICLES HAVING LOW ELECTRICAL CONDUCTIVITY OUTOF LIQUID MEDIA ONTO THE SURFACE OF A REFRACTORY METALLIC HEATERELEMENT; HEATING THE RESULTANT STRUCTURE IN AN OXYGEN FREE ATMOSPHEREUNTIL A SINTERED SOMEWHAT POROUS CERAMIC LAYER WELL BONDED TO SAIDSURFACE IS FORMED; COATING SAID CERAMIC LAYER WITH LACQUER TO SEAL OFFTHE PORES THEREOF; DEPOSITING MATALLIC PARTICLES OUT OF LIQUID MEDIAONTO THE CERAMIC LAYER; AND HEATING THE RESULTING STRUCTURE IN AN OXYGENFREE ATMOSPHERE UNTIL A SINTERED METALLIC SHEATH IS FORMED, SAID SHEATHBEING WELL BONDED TO SAID CERAMIC LAYER AND SEALING OFF THE PORESTHEREOF.